The present invention relates to printing screens, often alternatively referred to as stencils or masks, for use in the screen printing of workpieces, typically electronic substrates, such as circuit boards and components, frames therefor, and printing screen units.
In one known printing screen unit, the printing screen comprises a thin, stiff metal sheet, typically of stainless steel or brass, which includes a plurality of printing apertures which define a pattern for the printing of a printing medium, typically solder paste. The printing screen is mounted to a rectangular frame by a flexible, perforate sheet, typically a woven mesh of polypropylene or stainless steel strands, which acts to tension the printing screen.
Whilst the above-described printing screen unit has been well used, the unit does have a number of drawbacks, notably in being expensive to fabricate, and not allowing for the printing screen to be detached from the frame without permanently damaging the perforate sheet.
In another, more recent printing screen unit, the printing screen is a demountable printing screen which comprises a thin, stiff metal sheet, typically of stainless steel, which includes a plurality of printing apertures which define a pattern for the printing of a printing medium, typically solder paste, and a plurality of fixing apertures at opposite edges thereof. The printing screen is mounted to a frame by mechanical tensioning mechanisms which engage the fixing apertures in the printing screen and act to tension the printing screen, with the tensioning force typically being provided by hydraulic, pneumatic or spring means.
WO-A-93/25061 discloses one such printing screen unit in which the printing screen includes a plurality of elongate slots, as the fixing apertures, along each of opposite edges thereof, and the tensioning mechanisms each include a set of teeth which engage the slots at respective ones of the opposite edges of the printing screen to tension the printing screen, with the tensioning mechanisms being configured to draw respective ones of the opposite edges of the printing screen over respective supporting edges of the frame.
GB-A-2364961 discloses another such printing screen unit in which the printing screen includes a plurality of first, fixing apertures along each of opposite edges thereof which are engaged by respective tensioning mechanisms, and a plurality of second, flexing apertures along each of the opposite edges thereof which extend inwardly of the respective fixing apertures and provide for regions of increased flexibility, with the tensioning mechanisms being configured to draw respective ones of the opposite edges of the printing screen over respective supporting edges of the frame.
These printing screen units, in not requiring the permanent attachment of a printing screen to a frame, advantageously allow for printing screens to be stored separately and mounted on frames as and when required, thereby being cheaper and facilitating use.
These printing screen units do, however, still suffer from a number of significant drawbacks. In particular, the edges of the printing screens are required to be more flexible than the main body thereof, and, as such, are more prone to damage during handling and mounting. This is particularly the case as the printing screens are stored as unsupported sheets. Also, as the printing screens are very thin, typically having a thickness of from about 0.1 to 0.25 mm, the edges thereof are razor sharp, which can lead to personal injury to a user if not handled with utmost care. Further, the printing screens are restricted to being fabricated from materials which have the necessary strength to allow for direct tensioning by tensioning mechanisms. Yet further, the printing screens require specific manufacturing techniques, such as etching, electroforming and punching, and thereby limit the scope for mass fabrication.